This doesn't directly answer the question, but I think you may be overestimating the weight carried/worn by a knight.

The average sword weighed somewhere around 3lbs, an M16 with full ammo weighs about 9lbs.

A suit of full-plate would generally weigh less than 60lbs (sometimes significantly less), such armor was also custom fitted and designed to be relatively easy to wear with little strain. A full set of modern combat gear tops 60lbs, and is "one-size fits most".

"Medieval times' is to big a timeframe to answer really. In early medieval period 'knights' would be nobility. While knowledge of nutrition was poor most nobility had a strong diet of meat. While poor in vitamins it's pretty great for getting muscle. They also had less plate in this time and relied more on mail and gambersons. For mail I want to point out it's worn on the hips with a belt and not hanging from the shoulders. That really helps with the strain.

In the later period most 'knights' were in fact man at arms, professional warriors not of noble birth. While plate was dominant at that time plate is form fitted to the wearer. It's not heavier then what a modern infantry man wears.

"In Afghanistan, soldiers routinely carry loads of 130 to 150 pounds for three-day missions, said Jim Stone, acting director of the soldier requirements division at the Army Infantry Center at Fort Benning, Ga. In Iraq, where patrols are more likely to use vehicles, loads range from 60 to nearly 100 pounds, he said."

http://www.washingtonpost.com/wp-dyn/content/article/2009/01/31/AR2009013101717.html That's double a full set of plate. Of course you still got a weapon to wield. But most weapons are pretty light, less then a modern rifle and perfectly balanced. A well balanced weapon isn't exhausting to wield.

However full plate wearers did have one problem to overcome, heat. Wearing undercloths, a gamberson, some mail and plate was a pretty hot combination so I see over-heating being more of a problem then weight.

As for cleaving, you don't cleave with a sword. Against a user in plate you'll be stabbing with thin blades or crushing with a mace. For a mace you'll be keeping momentum and not ending your swing while the stabbing will be quick. Getting knocked down was a death sentence, beside that getting up wasn't that hard. Just laying there opened so many openings for a stab with a dagger.

While yes wielding a melee weapon is more tiring on the arms I doubt combat was that much more physically demanding back then for a 'knight'. Modern soldiers carry around lots of gear, a M249 weights a lot, a WW2 browning MG even more. Preferably a duel/fight lasts as little moves as possible, you don't keep hitting the other guy till one drops like a boxing match. But I can't give you an exact timeframe.

This might interest you, while not being a direct answer it does go on about the weight of armor and the comfort of wearing it. http://www.reddit.com/r/AskHistorians/comments/275iz3/in_shows_like_game_of_thrones_characters_wear/